1. Field of the Invention
The present invention generally relates to an optical recording medium for reversibly changing a state by irradiation with light beams to record information. More specifically, the invention relates to a phase change optical recording medium which has a change of state wherein the atomic arrangement of a thin film for holding recording changes between amorphous and crystalline arrangements.
2. Related Art
A typical phase change optical recording film has an amorphous atomic arrangement when its portion heated to its melting point or higher to melt is rapidly cooled. When the phase change optical recording film is held in a crystallized temperature range below the melting point for a predetermined time or longer, it is crystallized if its initial state is amorphous, whereas it remains crystal if its initial state is crystal. Since the intensity of reflected light from an amorphous region is different from the intensity of reflected light from a crystalline region, the principle of a phase change optical recording medium is that the intensity of reflected light is converted into the intensity of an electric signal to be analog-to-digital converted to read information.
There are two methods for increasing the amount of information capable of being recorded in a single recording medium, i.e. a recording capacity. One method is a method for scaling down the pitch of recording marks in track directions. In this method, if the degree of scale down proceeds, the pitch becomes smaller than the size of a reproducing light beam, so that there are some cases where two recorded marks are temporarily included in a reproducing beam spot. If the recording marks are sufficiently spaced from each other, a regenerative signal can be greatly modulated to obtain a signal having a large amplitude. However, if the recording marks are close to each other, the amplitude of the signal is small, so that errors are easy to occur when the signal is converted into digital data.
Another method for improving a recording density is a method for narrowing the track pitch. This method can increase the recording density without being greatly influenced by the decrease of the signal strength due to the scale down of the mark pitch. However, in this method, there is a problem in that, in a region in which a track pitch is substantially equal to or smaller than the size of a light beam, there is caused a so-called cross erase wherein information on a certain track deteriorates when a writing or erasing operation is carried out in an adjacent track.
Two causes of cross erase are considered. One cause is that, when adjacent tracks are irradiated with a beam, the light intensity of the beam at the bottom edge thereof within a subject track is not small, so that the recording mark of the track is deteriorated by only the effect of irradiation with light. The other cause is that, when adjacent tracks are heated by a light beam, generated heat is transferred to the tracks by heat transfer in film in-plane directions, so that the shape of a recording mark is deteriorated by the influence thereof. Since the influence of the cross erase due to the latter influence can be reduced by decreasing heat transfer in the film in-plane directions, it has been devised to reduce the cross erase by more greatly promoting heat conduction in directions perpendicular to the plane of a recording film than in the in-plane directions by forming a so-called rapid-cooling structure by arranging a film having a large conductivity and/or heat capacity in the vicinity of a recording film.
For example, a conventional phase change optical recording medium shown in FIG. 10 comprises a substrate 301, a metal reflecting film 302 formed on the substrate 301, a transparent dielectric film 303 formed on the metal reflecting film 302, a recording film 304 formed on the dielectric film 303, a transparent dielectric film formed on the recording film 304, and a cover layer 306 formed on the dielectric film 305. That is, the dielectric film 303 is arranged between the recording film 304 and the metal reflecting film 302 for ensuring a signal strength by the reflection of light, and the dielectric film 303 is formed so as to be relatively thin, so that heat generated by the recording film 304 can easily escape to prevent heat from being transferred in the film in-plane direction. As the thickness of the dielectric film 303 decreases, heat transfer in direction perpendicular to the plane of the film can be promoted to improve cross erase.
However, if the dielectric film 303 is too thin, heat transfer to the reflecting film 302 starts simultaneously with heating due to laser beams during recording, so that there is a problem in that the temperature rise of the recording film 304 is insufficient, whereby the temperature of an area required to form a recording mark does not reach the melting point. In addition, if a laser power of erase level is applied, the mark cools immediately after heating. Therefore, the temperature of the mark can not be held in a temperature range capable of crystallizing the mark for a sufficiently long time, so that there is a problem in that it is difficult to crystallize the mark, i.e. to carry out an erasing operation, thereby remarkably deteriorating the erasing rate.
Conversely, if the dielectric film 303 is too thick, there is no problem on the power margin and erasing rate with respect to laser beams during recording. However, as described above, heat transfer into the plane of the film is not only promoted to violently cause cross erase, but the cooling rate of the recording film 304 is slow. Therefore, there is a problem in that the region melt during recording is crystallized again without being amorphous, so that the formed mark is too small.
Japanese Patent Laid-Open No. 2000-215516 discloses that cross erase can be suppressed by including at least a recording layer, a top protective layer, an intermediate layer and a reflecting layer in order from a light incident side and by defining characteristics of the materials of the intermediate and reflecting layers. However, since this method does not sufficiently select the material of the intermediate layer and selects a material having a low thermal conductivity, it is difficult to carry out rapid cooling, so that this method can not sufficiently reduce cross erase.
Thus, the thickness and heat conduction characteristics of the dielectric film between the metal reflecting film and the recording film are required to simultaneously eliminate the problems on the power sensitivity in recording, cross erase, recrystallization and erasing rate. However, conventional means can not simultaneously satisfy all of them.